The invention relates to optical switching and free-space coupling of fiber optic waveguides in a single-mode fiber-optic transmission system. The invention finds application to micro-electromechanical systems (MEMS), but it is not so limited.
Four-axis detection schemes in fiber optic switch fabrics are known which use complex metrology systems to indirectly infer positioning of a laser beam of interest. One such technique, which is representative of the prior art, is described in U.S. Pat. No. 6,097,858, assigned to Astarte Fiber Networks of Boulder, Colorado. It uses forward- and reverse-facing metrology lasers (i.e., lasers located at input fibers and output fibers, respectively) and two-axis photoconductive sensors surrounding the input and output fibers to detect the beam alignment as measured by the metrology system. Since these augmented-metrology systems do not make direct use of the power signal they are attempting to maximize, their performance (ability to maximize the coupled power) is degraded by the unavoidable time-varying misalignment between the metrology system and the actual beam.
A Melles Griot active alignment system for fiber-optic coupling known as the NanoTrak Autoalignment system uses the measured output power and a synchronous-detection approach to null the errors in two angles using a coning motion in the two controlled axes of an optical mount. Conical scanning, while appropriate for detecting errors in two axes, is not well suited to a four-axis system addressed by the present invention or to systems with even greater degrees of freedom. The key limitation is that coning the first mirror at one frequency and the second mirror at a second frequency yields a coupled power response that contains oscillatory components at frequencies equal to the sums, differences, and first harmonics of the two frequencies, even when the alignment errors (ignoring coning angles) are zero. Thus, the double-coning approach cannot be used in a system requiring a constant output power.
What is needed is a technique for active alignment that overcomes the above limitations.
According to the invention, methods and apparatus are provided for detection and control of multiple-axis active alignment for a free-space-coupled single-mode fiber-optic transmission system that automatically optimizes the power coupled through the system. In a specific embodiment, a measurement of coupled power is made and detected error signals are used to control actuation via four axes of beam steering elements to null four generally orthogonal alignment errors (combinations of two lateral errors and two angular errors) of the beam between the input and output fibers. The four alignment errors are detected using a synchronous-detection approach. A feedback control system nulls the four errors.The theoretical basis as presented here for four-axis detection and control is sufficient for the general case. Therefore, the disclosure is to be understood to address the cases for applications of more or fewer axes than four.
The present invention related to alignment has application to synchronous detection. In the case of four-axis synchronous detection, a control system superimposes four distinct modes of oscillatory commands (dithers) as excitation signals on four nominal steering commands. These dithers, which themselves must be orthogonal, are specifically chosen to produce four corresponding time-orthogonal variations in measured coupled power at the dither frequencies that are proportional to the respective alignment errors. Specific examples of orthogonal mode variable signals for detection as disclosed herein are two sets of sine and cosine signals at two different frequencies. The invention will be better understood by reference to the following detailed description in connection with the accompanying embodiments.